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Structural Setting of the 1999 U.S.-Japan Nankai Trough 3-D Seismic Reflection
Survey G F Moore 1 (808-956-6854; gmoore@Hawaii.edu) A Taira 2 S Kuramoto 3 T H Shipley 4 N L Bangs 4 EW9907/8 Shipboard Scientific Party 1University of Hawaii, Dept. Geoplogy & Geophysics 1680 East-West Rd., Honolulu, HI 96822, United States 2University of Tokyo, Ocean Research Institute 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan 3Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305-8537, Japan 4University of Texas , Institute for Geophysics 4412 Spicewood Springs Rd., Austin, TX 78759-8500, United States As part of the recent 3-D seismic reflection survey of the Nankai Trough subduction zone, a long 2-D reflection line was collected through the middle of the 3-D box to establish the regional structural setting. The line extends from 40 seaward of the trench axis landward across the subduction complex to within 35 km of Muroto Point on Shikoku Island. Seaward of the trench axis buried basement topography of the Kinan Seamount chain is imaged as a broad ridge overlain by 0.7 sec (750 m) of sediment. Seaward of the ridge the sediment thickness increases to about 1.75 sec (2 km); the deepest approx. 1.0 sec of sediment onlaps the ridge, indicating that the ridge was formed before sedimentation began. The youngest 0.6 sec (700 m) of hemipelagic sediment is draped over the ridge and continues into the trench axis where it is buried by 0.3 sec (350 m) of trench turbidites. The trench strata and the upper hemipelagic deposits are stripped off the subducting plate and accreted. The accretionary complex is divided into several segments based on structural styles. Between the deformation front and the frontal thrust is a zone of diffuse structural thickening, the proto-thrust zone. The frontal section of the accretionary prism is characterized by 700-800 m-thick thrust slices. There are twelve major thrust packets with a thrust spacing of about 1.5-2.0 km. The thrusts sole into a well-defined decollement approximately 0.25-0.30 sec above the oceanic crust. Approximately 20 km landward of the frontal thrust is a major out-of-sequence thrust (OOST) that thickens the prism from 1.6 sec to 1.9 sec. A 6.25-km wide slope basin with about 0.6 sec of sediment is ponded behind this OOST. Strata within the basin are tilted landward, with dips that increase with depth, indicating continued relative uplift along the OOST during sedimentation. Approximately 30 km landward of the frontal thrust the thrust packets abruptly increase to at least 1 sec thickness, with thrust spacing increasing to 2-3 km. These thrust slices are formed of strata equivalent to the deeper section seaward of the Kinan Seamounts. The decollement cuts down closer to the oceanic crust beneath these thrust packets. Another major slope basin has formed above these thrust slices. Landward of this zone the prism is characterized by less distinct, landward-dipping reflections; the decollement is obscured by the seafloor multiple. |
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Last Modified: October 8, 1999
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